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I'm not sure what burnt means. If you are using a clay formulated for raku, which is usually a low fire clay, and you reached cone 8 your pots should be slumped or puddled all over your kiln. I would think that you will have a lot more difficulty getting enough glaze on the pieces to form glass and getting the smoked black on the unglazed areas than problems with cracking the pots. It is possible to refire cone 10 electric glaze fired pots in a raku kiln to reduce them so I wouldn't think firing to raku temperatures with pots already fired way past the normal firing temperature for raku would cause that big a problem with cracking. I know people who bisque to 04 for raku and then fire the glaze to a slightly lower temperature and they come out even though the clay was vitrified in the bisque. Simplest way to tell is try to fire them and see what happens.

Mike

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Actually, good raku clay should be a high fire clay, cone 6 upward. It stays open becasue it does not vitrify. If you fired to cone 8, your clay is probabluy a cone 10 clay and, therefore, close to vitrification at cone 8. You may still be able to get some light carbon markings and reduction that could be very interesting and subtle. It would be interesting to try splashes of slip resist and heavily reduce in sawdust to see what happens. I am imagining lovely cloudy greys. If you do dercide to give it a try, let us know and post pictures. Good luck.

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I have refired a number of pieces that were made with raku clay. The trouble usually comes in the 650 to 900 degree F. range. you might give some thought to salt fuming or pit firing. Please let us know how it turns out.

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I bisque fire my Raku pieces to cone 06 - basically the same temp as the Raku final fire. Then glaze/tape/naked raku or whatever other decorating technique, and do the final Raku fire at around 1800 F for most pieces before reduction. Not sure if this temp applies to all Raku clays, but at least it seems to work well for the Raku clay I'm using.